Light olefins, defined herein as ethylene and propylene, are important commodity petrochemicals useful in a variety of processes for making plastics and other chemical compounds. Ethylene is used to make various polyethylene plastics, and in making other chemicals vinyl chloride, ethylene oxide, ethyl benzene and alcohol. Propylene is used to make various polypropylene plastics, and in making other chemicals such as acrylonitrile and propylene oxide.
The petrochemical industry has known for some time that oxygenates, especially alcohols, are convertible into light olefins. The preferred conversion process is generally referred to as an oxygenate to olefin (OTO) reaction process. Specifically, in an OTO reaction process, an oxygenate contacts a molecular sieve catalyst composition under conditions effective to convert at least a portion of the oxygenate to light olefins. When methanol is the oxygenate, the process is generally referred to as a methanol to olefin (MTO) reaction process. Methanol is a particularly preferred oxygenate for the synthesis of ethylene and/or propylene.
Depending on the respective commercial markets for ethylene and propylene, it may be desirable to vary the weight ratio of ethylene to propylene formed in an OTO reaction system. It has recently been discovered, however, that although percent conversion may vary with a change in reaction conditions, e.g., temperature or pressure, the selectivity of a methanol-containing feedstock for ethylene and propylene in an OTO reaction system generally remains constant with changes in reaction conditions. Thus, the need exists in the art for a process for varying the ratio of ethylene to propylene formed in an OTO reaction system.